Change in Method of Accounting for Inventory Valuation - Summary of Current Year Financial Statement Line Items If Company Does Not Changed Method of Accounting for U.S. Inventories from LIFO to FIFO (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2018
Income Statement [Abstract]
Cost of Sales	[1]	$ 404,561	$ 408,137
Gross Profit	[1]	84,609	91,198
Operating Income	[1],[2]	29,738	41,283
Income Before Provision for Income Taxes	[1],[2]	31,030	39,292
Provision for Income Taxes	[1]	6,052	7,347
Net Income	[1],[2]	24,978	31,945	[3]
Net Income Attributable to Stepan Company	[1],[2]	$ 24,984	$ 31,952
Net Income Per Diluted Common Share Attributable to Stepan Company	[1],[2]	$ 1.07	$ 1.37
Balance Sheet [Abstract]
Inventories		$ 215,028			$ 231,528
Other Non-Current Assets	[2]	10,332			9,549
Total Assets	[2]	1,526,000			1,514,614
Deferred Income Taxes	[2]	24,158			24,961
Retained Earnings	[2],[4]	861,773			837,107
Total Liabilities and Equity	[2]	1,526,000			1,514,614
Statement of Cash Flows [Abstract]
Net Income	[1],[2]	24,978	$ 31,945	[3]
Deferred Income Taxes	[2]	(1,857)	756
Changes in assets and liabilities:
Inventories	[2]	16,231	(10,621)
Effect of Change [Member]
Income Statement [Abstract]
Cost of Sales			(1,628)
Gross Profit			1,628
Operating Income			1,628
Income Before Provision for Income Taxes			1,628
Provision for Income Taxes			399
Net Income		(1,132)	1,229
Net Income Attributable to Stepan Company			$ 1,229
Net Income Per Diluted Common Share Attributable to Stepan Company			$ 0.06
Balance Sheet [Abstract]
Inventories		(32,863)			31,363
Other Non-Current Assets		1,483			(1,415)
Total Assets		(31,380)			29,948
Deferred Income Taxes		(6,589)			6,289
Retained Earnings		(24,791)			23,659
Total Liabilities and Equity		(31,380)			$ 29,948
Statement of Cash Flows [Abstract]
Net Income		(1,132)	$ 1,229
Deferred Income Taxes		(368)	399
Changes in assets and liabilities:
Inventories		1,500	$ (1,628)
As Computed under LIFO [Member]
Income Statement [Abstract]
Net Income		23,846
Balance Sheet [Abstract]
Inventories		182,165
Other Non-Current Assets		11,815
Total Assets		1,494,620
Deferred Income Taxes		17,569
Retained Earnings		836,982
Total Liabilities and Equity		1,494,620
Statement of Cash Flows [Abstract]
Net Income		23,846
Deferred Income Taxes		(2,225)
Changes in assets and liabilities:
Inventories		17,731
ProForma [Member] | Effect of Change [Member]
Income Statement [Abstract]
Cost of Sales		1,500
Gross Profit		(1,500)
Operating Income		(1,500)
Income Before Provision for Income Taxes		(1,500)
Provision for Income Taxes		(368)
Net Income		(1,132)
Net Income Attributable to Stepan Company		$ (1,132)
Net Income Per Diluted Common Share Attributable to Stepan Company		$ (0.05)
Statement of Cash Flows [Abstract]
Net Income		$ (1,132)
ProForma [Member] | As Computed under LIFO [Member]
Income Statement [Abstract]
Cost of Sales		406,061
Gross Profit		83,109
Operating Income		28,238
Income Before Provision for Income Taxes		29,530
Provision for Income Taxes		5,684
Net Income		23,846
Net Income Attributable to Stepan Company		$ 23,852
Net Income Per Diluted Common Share Attributable to Stepan Company		$ 1.02
Statement of Cash Flows [Abstract]
Net Income		$ 23,846

[1] The 2018 amounts for the noted line items have been changed from the amounts originally reported as a result of the Company’s first quarter 2019 change in method of accounting for U.S. inventory valuation from the last in, first out (LIFO) basis to the first in, first out (FIFO) basis.

[2] The 2018 amounts for the noted line items have been changed from the amounts originally reported as a result of the Company’s first quarter 2019 change in method of accounting for U.S. inventory valuation from LIFO to FIFO.

[3] The retained earnings and net income amounts for the noted line items have been changed from the amounts originally reported as a result of the Company’s first quarter 2019 change in method of accounting for U.S. inventory valuation from LIFO to FIFO.

[4] The 2019 amounts for the noted line items include an adjustment related to the Company’s first quarter 2019 adoption of ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.